Sudden Chain Slack

Came home from a long slab ride and chain was so loose I pulled it off the rear sprocket with one hand!

The wheel has not moved. Stock chain/sprockets are fairly worn but not too much (far as I can tell).

Why so suddenly slack to that degree?

Also, just out of curiosity, what would have happened had that sucker came flying off during my ride?

Probably because the chain needs to be replaced as the sealed-in lube has been spent.
I take that as a hint to do so (chain and both sprockets).

If the chain derails while riding, more than likely, the chain bunches-up between the front sprocket and engine crankcase, resulting in a hole punched through the crankcase in the worst case, and a pain in the ass in the best case.

When my stock chain got replaced it was so worn that I had to adjust it every 30 miles. The front sprocket was also missing 3 teeth.

That was at less than 7K miles of 100% on road/track use.

The stock chains seems to have bad quality: my chain broke after 300km (one week.after i have buy it).
I jumped (maybe 2m), and when i reached the ground, i hear a loud bang. The bike didn't drive after this...
The chain was 50m behind the bike.
First i didn't notice, that i have no chain. My friend said after a few minutes :"hey you have no chain, lets go looking where it is.."

But nothing else was broken, only the plastikcap around the sprocket and the chain.

The Yamaha Dealer said, that this is not saved by the warenty, because the wrr is no offroadbike like the wrf... Thought he was joking..